The present invention relates to a composite polymer membrane, a method for producing the same and a solid polymer electrolyte membrane.
In recent years, fuel cells have occupied an important position as next generation type clean energy sources. Of these fuel cells, a solid polymer electrolyte type fuel cell is one in which both anode and cathode electrodes are each arranged across a solid polymer electrolyte membrane intervening therebetween. For example, in the case of a direct methanol type fuel cell (hereinafter referred to as a xe2x80x9cDMFCxe2x80x9d) in which methanol is used as a fuel, methanol is supplied to the anode side, and oxygen or air to the cathode side, thereby allowing electrochemical reaction to occur to generate electricity. Solid polymer electrolyte membranes having high proton conductivity have been developed for retaining the characteristics of their high output and high energy density, and for obtaining small-sized, lightweight fuel cells. The solid polymer electrolyte membrane used in the DMFC is required to have the barrier property to fuel methanol, that is to say, reduced permeability (cross-over) of fuel methanol from the anode side of the membrane to the cathode side thereof.
Previously, hydrated membranes of perfluorosulfonic acid polymers such as Nafion (trade name) manufactured by E. I. du Pont de Nemours and Company) have generally been used as the solid polymer electrolyte membranes. The structure of Nafion is represented by the following general formula (I): 
The above-mentioned hydrated membranes of perfluoro-sulfonic acid polymers have high proton conductivity, and the proton conductivity is exhibited by the generation of a channel structure caused by hydration (conduction of hydrated protons) That is to say, the conduction of protons takes place through water as a medium in the hydrated membranes of perfluorosulfonic acid polymers, so that a specified amount of water exists in the hydrated membranes. Accordingly, methanol having high affinity with water easily passes through the membranes, so that the hydrated membranes of perfluorosulfonic acid polymers have a limitation with regard to the methanol barrier property.
As means for reducing the cross-over of methanol through the hydrated membranes of perfluorosulfonic acid polymers, composites of different kinds of materials based on the hydrated membranes of perfluorosulfonic acid polymers may be mentioned. However, the above-mentioned composites have significantly lower proton conductivity than the essential one of the hydrated membranes of perfluorosulfonic acid polymers.
The present invention has been made against a background of the current problems of the hydrated membranes of perfluorosulfonic acid polymers as the solid polymer electrolyte materials as described above.
An object of the invention is to provide a composite polymer membrane.
Another object of the invention is to provide a method for producing the same.
A further object of the invention is to provide a solid polymer electrolyte membrane excellent in proton conductivity and methanol barrier property.
The present inventors have conducted intensive investigation for attaining the above-mentioned objects. As a result, the inventors have discovered that a solid polymer electrolyte membrane excellent in proton conductivity and methanol barrier property is obtained by allowing aniline to be adsorbed by an acidic group-containing polymer membrane, and subjecting aniline to oxidative polymerization to obtain a polyaniline-containing polymer membrane, thereby preparing a composite membrane, thus completing the invention.
The invention provides a composite polymer membrane in which a polyaniline-containing polymer membrane is formed on a surface of an acidic group-containing polymer membrane.
Aromatic rings in repeating units of the above-mentioned polyaniline may have substituents.
It is preferred that the above-mentioned acidic group-containing polymer is a sulfonic acid group-containing water-absorptive polymer.
The above-mentioned acidic group-containing polymer preferably has proton conductivity.
The above-mentioned acidic group-containing polymer is preferably a perfluorosulfonic acid polymer.
It is preferred that the above-mentioned composite membrane has proton conductivity by hydration.
Further, the invention provides a method for producing the above-mentioned composite membrane, which comprises allowing aniline to be adsorbed by an acidic group-containing polymer membrane, and subjecting the aniline to oxidative polymerization to obtain a polyaniline-containing polymer membrane.
In the above-mentioned method, it is preferred that the acidic group-containing polymer membrane is subjected to equilibration treatment in deionized water or an aqueous solution of an acid before the adsorption of aniline to allow an acid to be contained in the polymer membrane.
In the above-mentioned method, it is preferred that the thickness of the polyaniline-containing polymer membrane and the concentration of the polyaniline contained in the composite membrane are increased by increasing the concentration of the aqueous solution of the acid.
Still further, the invention provides a solid polymer electrolyte membrane comprising the above-mentioned composite membrane.